This is normally an indication that you are trying to use both the sendmail server and the transparent SMTP proxy service at the same time.
Because sendmail performs a wildcard bind on each zone it is configured to use, you can't then use the SMTP proxy (because it will try to bind to the same point). Of course in v8 these services are called "applications" rather than proxies, but they are essentially doing the same thing.
Historically, I have seen this exact error where one of our customer's has tried to create an SMTP proxy rule, without realising that sendmail is running. They find the rule doesn't work and decide to re-boot the Firewall. When the firewall comes back up the SMTP proxy service wins the race to grab hold of TCP port 25 and then sendmail is unable to start and it is at this point these messages start appearing in /var/log/maillog.
While I've never done it myself, it may be possible to edit one of the sendmail configuration files so that it only binds to a specific IP address in each zone - allowing you keep sendmail running but to then pass transparent SMTP traffic via an alias IP address. The other option is to pass all mail via the firewall and let sendmail do the job of delivering it for you.